Add advice on choosing between scheduler and threading.Timer().

Doc/library/sched.rst

@@ -41,6 +41,31 @@ Example::
    From print_time 930343700.273
    930343700.276
 
+In multi-threaded environments, the :class:`scheduler` class has limitations
+with respect to thread-safety, inability to insert a new task before 
+the one currently pending in a running scheduler, and holding-up the main
+thread until the event queue is empty.  Instead, the preferred approach
+is to use the :class:`threading.Timer` class instead.
+
+Example::
+
+    >>> import time
+    >>> from threading import Timer
+    >>> def print_time():
+    ...     print "From print_time", time.time()
+    ...
+    >>> def print_some_times():
+    ...     print time.time()
+    ...     Timer(5, print_time, ()).start()
+    ...     Timer(10, 1, print_time, ())
+    ...     print time.time()     # executes before the time-delay events 
+    ...
+    >>> print_some_times()
+    930343690.257
+    930343690.301
+    From print_time 930343695.274
+    From print_time 930343700.273
+
 
 .. _scheduler-objects: